FRUITS AND COLONISATION BY SEED. 47 



the distribution of Cotton by man ; for, as long as man 

 continues to use clothes, no other textile is ever likely 

 to replace it. 



The third series is even more interesting, as the idea 

 of wmgs or aeroplanes lends itself to a greater variety 

 of treatment. These have been recently classified by 

 Dingier (see p. 49). Sometimes these wings form a 

 parachute, in other cases they act like sails. The seed 

 may be flat, with a continuous wing as in many Lilies, 

 or there may be three wings running longitudinally, as 

 in the fruits of Polygonaceae. The Ash has but one broad 

 wing. The arrangement in the Sycamore fruit and Pine 

 seed is one of the most beautiful, and at the same 

 time, perhaps the most difficult to understand of 

 any of them. Each carpel of the former ends in 

 a long, slightly twisted blade much broader than the 

 fruit. There are generally two of these carpels 

 attached together. When they begin to fall, these 

 flat wings press upon the air, and, in consequence of 

 the position in which the weight is slung, moments 

 are set up which cause the system to twirl round 

 and round as it falls. The motion is curiously 

 similar to that of the screws of a steamer, but whilst 

 in the latter the turning of the screw causes the 

 motion of the boat, in the Sycamore the motion of 

 the fruit as it falls causes the twirling of the wings. 

 This, however, is not a scientifically exact explanation, 

 for which advanced mathematics are required. A very 

 neat little parachute is found in the Sea Pink or Thrift 

 {Armeria vulgaris')^ where the fruit is surrounded by a 

 thin papery funnel intended to catch the wind. In 

 Dipsacus, a very similar funnel-like parachute occurs, 

 but it is formed from the bracts, and not, as in Armeria^ 

 from the united sepals. 



